Nanocomposite technologies based on the dispersion of layered materials such as layered silicates in a polymeric matrix are expected to become extremely important in the plastics industry over the next few decades. They offer huge opportunities in a broad range of markets through unprecedented enhancement of physical properties, pushing the performance envelope well beyond the domain of known composite technologies. This is because dispersed exfoliated layered silicates consist of approximately 1 nm thick platelets with aspect ratios that can exceed 2,000, leading to a much greater specific contact area with a polymer matrix than for an equivalent volume fraction of a conventional filler. Polypropylene nanocomposites with nearly twice the stiffness and a significantly higher softening temperature (heat deflection temperature) compared with the neat resin are achievable with no loss in surface quality and a specific gravity less than 0.95. At the same time substantial increases in adhesion are anticipated, with adhesive bond strength increasing 7-fold in epoxy nanocomposites, for example. Barrier properties, flame retardance or electrical conductivity may also be improved (Ruiz-Hitzky, et al. Adv. Mater. 7, (1995) 180; Kato et al. Clays and Clay Mater. 5, (1993) 1694), and the presence of exfoliated silicate layers can result in increased viscosity and elasticity in fluids, including polymer melts. Improvements in properties such as mechanical strength, stiffness and softening temperature have been disclosed in U.S. Pat. No. 4,739,007; U.S. Pat. No. 4,810,734; U.S. Pat. No. 5,385,776.
Considerable work has consequently been devoted to the development of new synthetic methods for combining polymers and layered materials, such as smectite clays, and sodium montmorillonite in particular, by in situ polymerization of intercalated monomeric precursors (Alexandre and Dubois, Mat. Sci. & Eng. 28, (2000) 1). Nevertheless, for practical and economic reasons, fabricating such composites by melt or solution processing of readily available and cheap materials with minimal modification is a primary goal. There has been particular interest in identifying conditions or molecular characteristics that favor exfoliation of the layered materials in the polymer matrix (Singh and Balazs, Polymer International 49, (2000) 469; Ginzburg et al., Macromolecules 33, (2000) 1089; Zhulina et al., Langmuir 15, (1999) 3935), since the consequent percolation of interlayer contacts at very low loadings is thought to be determinant for many physical properties (Kojima et al., J. Mater. Res. 6, (1993) 1185; Messersmith and Giannelis, Chem. Mater. 6, (1994) 1719; Lan and Pinnavaia, Chem. Mater. 6, (1994) 2216). As described in WO93/04118, the incorporation of individual platelets of a highly hydrophilic exfoliated smectite clay into a polymer can be achieved by converting the hydrophilic clay into an organophilic clay by adsorption of a silane coupling agent or an onium cation, which is compatible with the polymer matrix and significantly increases the interlayer spacings. Intercalation of the organic polymer molecules between the organophilic silicate layers substantially increases the interlayer spacings still further, and the layers can be separated by high shear mixing, for example.
The most widely used swellable layered materials have negative charges or basic sites on the layers, with a commensurate number of exchangeable cations in the interlayer spaces. These include smectite clay minerals such as montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, magadiite, and kenyaite. Hectorite and montmorillonite, with between about 20 basic sites and about 150 basic sites per 100 g material are among the most suitable layered materials for exfoliation or intercalation in polymer matrices. However, the exchangeable inorganic cations such as sodium or calcium ions characteristic of the native clay, the interlayer spacings do not usually exceed 0.4 nm in the dry state in air and the interlayer cohesive energy is generally too strong to permit facile exfoliation and direct dispersion into a polymer or a polymer precursor matrix.
Another important factor in facile dispersion of exfoliated layers into a polymer matrix is the strength of specific interactions between the polymer and the layers. For example, polypropylene (PP) is relatively inert and itself shows little affinity for smectite clays. However, melt intercalation and exfoliation have been reported in PP/smectite clay mixtures modified by grafting maleic anhydride to the PP, increasing its polarity and hence the strength of its interactions with the layer surfaces (Hasegawa, N. et al., J. Appl. Poly. Sci. 67, (1998) 87). In the case of highly functional polymers, including highly polar polymers and ionomers, for example, intrinsically strong bonding between the polymer and the silicate layers can occur through ion exchange, electrostatic complexes, direct hydrogen bonding or hydrogen bonding via water bridges in aqueous solution, chelation, dipole-dipole interactions and dispersive forces. In native smectite clays, electronegative species including oxygen in hydroxyl groups or sulfur in thiol groups may sorb to interlayer cations, such as sodium ions in sodium montmorillonite. The electronegativity should ideally be 2 or more on the Pauling scale for strong sorption to occur.
Strong interactions between a polymer and the layers in a smectite silicate nevertheless do not necessarily lead to facile exfoliation, since the presence of large numbers of functional groups on individual linear polymer molecules can give rise to bridging effects. Thus, aqueous suspensions of linear water soluble polymers such as poly vinyl pyrrolidone, poly vinyl alcohol and poly ethylene oxide have been reported to result in intercalation of smectite clays with relatively small silicate inter-gallery layer spacings, unsuitable for dispersion by high shear mixing (Ogata et al., J. Appl. Polym. Sci. 66, (1997) 573; Levy and Francis, J. Colloid Interface Sci. 50, (1975) 442; Greenland, J. Colloid Sci. 18 (1963) 647). Melt intercalation with polymers of this type has also been reported, but again leads to relatively small increases in gallery spacing, even after long heat treatment times (Vaia et al. Chem. Mater. 5, (1993) 1694).